1. Field of the Invention
The present invention relates generally to an apparatus and method for controlling a digital automatic gain controller (AGC), and in particular, to an apparatus and method for controlling a digital AGC by estimating the power of an uplink signal in the next frame in an orthogonal frequency division multiple access (OFDMA) communication system, wherein power changes significantly from frame to frame.
2. Description of the Related Art
In general, an analog signal is converted to a digital signal through sampling and quantization in an analog-to-digital converter (ADC). The digital signal is obtained by quantizing the analog signal into quantization levels preset for the ADC. More specifically, the digital signal is bit data representing the range of the analog signal preset for the ADC.
If the amplitude of an ADC input signal is in the preset range, some significant bits, which represent the analog signal, are varying, while the remaining bits are unchanged. In this case, transmission of the unchanged bit values adds up to the amount of transmission data. Consequently, the amplitude of the digital signal is controlled by a digital AGC, thereby taking only the effective bits. This is equivalent to keeping the amplitude of the analog signal constant in an analog AGC and taking significant bits.
FIG. 1 is a block diagram of a conventional AGC apparatus. Referring to FIG. 1, the conventional AGC apparatus includes a digital AGC 101, an effective bit detector 102, a power measurer 103, and a gain calculator 104. The power measurer 103 measures the average power of a signal received for a predetermined time period. The gain calculator 104 calculates a gain for the digital AGC 101 based on the average power measurement. The digital AGC 101 applies the gain to the received signal so that its output signal has desired amplitude. During this operation, the digital AGC 101 adjusts the amplitude of the data by shifting the input digital signal to the left or to the right.
The effective bit detector 102 eliminates unnecessary, least significant bits (LSBs) from the data stream received from the digital AGC 101. That is, the effective bit detector 102 takes only the effective bits having data information from the digital signal input to the digital AGC 101. The digital signal output from the effective bit detector 102 is provided to a channel card, for channel demodulation.
As described above, the conventional communication system calculates a gain for the digital AGC based on the average power of a signal received from a predetermined time period. However, because an OFDMA communication system using multiple carriers performs scheduling on a frame-by-frame basis, subcarriers allocated to each user and the number of the allocated subcarriers vary from frame to frame. Therefore, power changes greatly between frames. Because the conventional AGC apparatus illustrated in FIG. 1 is designed to control the gain of the digital AGC based on the power of the previous received signal under the assumption of a low power variation rate, its application to the OFDMA communication system makes it difficult to detect effective bits with accuracy.